Top-level `await` (Stage 3)

Champion: @MylesBorins https://twitter.com/MylesBorins/status/999005736914087936

> Presented as Stage 1 at [Jan 2018](https://github.com/babel/proposals/issues/37#issuecomment-360279595)
> Moved to Stage 2 at [May 2018](https://github.com/babel/proposals/issues/42#issuecomment-391035527)

Spec repo: https://github.com/tc39/proposal-top-level-await